1. Field of the Invention
The present invention relates to broadband communications, and in particular to a hybrid circuit which couples a broadband modem to a plain-old-telephone service (POTS) line.
2. Related Art
The evolution of the public switched telephone network from an analog to a digital network, and the introduction of the Internet have stimulated the growth of high bandwidth services. These services include such things as interactive gaming, interactive websites incorporating video and audio, video conferencing, online videos, and downloading of large files. Increasingly, however, consumers are demanding higher quality and faster broadband services.
Service providers deliver these high bandwidth services to consumers using a variety of delivery technologies, such as cable modems and digital subscriber line (DSL) modems. A plethora of DSL technologies exist, such as symmetric subscriber line (SDSL), asymmetric DSL (ADSL) and very high rate DSL (VDSL). Common to each DSL approach is that the broadband modems use the existing telephone infrastructure, including the so called “last mile” of the network, which is the leg from a central office to a customer premise (e.g., home or office).
ADSL, for example, is a high speed duplex broadband transmission technique used to establish a broadband connection over ordinary telephone lines using two broadband modems placed between a customer premise and a central office. The broadband modem at the central office is, in turn, connected to routers that are connected to the Internet or other packet switched network. For all DSL applications an approach must be used to couple receive and transmit modem signals from the telephone line to the broadband modem, and to separate the receive and transmit modem signals. A hybrid circuit is often the approach used. Typically, a transformer in the hybrid circuit provides gain for the transmit modem signal, and attenuates the receive modem signal by the same magnitude as the gain provided for the transmit modem signal.
The evolution of integrated circuit technology and the need to decouple modem transmit and receive signals presents a challenge. In particular, industry technical specifications for ADSL specify a peak transmit voltage that is high, fixed at sixteen volts peak-to-peak, whereas the supply voltages for advanced integrated circuits keep decreasing. Simply increasing the turns ratio of a transformer within a hybrid circuit to achieve the high voltage needed for the modem transmit signal will cause more attenuation of the modem receive signal. This approach will severely limit performance because the signal level of the modem receive signal would be significantly lowered and, as a result, more errors would occur in attempting to process the modem receive signal. Current approaches to this problem are to have either a separate high voltage capable linedriver integrated circuit within a broadband modem or to have a specialized process that combines a high voltage capable linedriver circuit with a broadband modem. These approaches, however, introduce additional costs and complexity into broadband transmission hardware.
What is needed is a hybrid circuit that can effectively decouple modem transmit and receive signals without introducing unnecessary costs or other performance impairments.